1,802 research outputs found
Large Area Silicon Sheet by EFG
Progress made in the development of EFG ribbon growth is discussed. Specific areas covered include: (1) demonstration of multiple growth for ribbons 5 cm wide in runs of 12 and 20 hours duration; (2) a single cartridge crystal growth station was built expanding observational capacity by virtue of an anamorphic optical-video system which allows close observation of the meniscus over 7.5 cm wide, as well as video taping of the ribbon growth process; (3) growth station no.1 achieved reproducible and reliable growth of 7.5 cm wide ribbon at speeds up to 4 cm/min; (4) introduction of the 'mini cold shoe'; (5) increases in cell efficiency due to interface shaping using the 'displaced die' concept; and (6) clarification of the role of gaseous impurities in cartridge furnaces and stabilization of their destabilizing influence on growth
Astrometric Effects of a Stochastic Gravitational Wave Background
A stochastic gravitational wave background causes the apparent positions of
distant sources to fluctuate, with angular deflections of order the
characteristic strain amplitude of the gravitational waves. These fluctuations
may be detectable with high precision astrometry, as first suggested by
Braginsky et al. in 1990. Several researchers have made order of magnitude
estimates of the upper limits obtainable on the gravitational wave spectrum
\Omega_gw(f), at frequencies of order f ~ 1 yr^-1, both for the future
space-based optical interferometry missions GAIA and SIM, and for VLBI
interferometry in radio wavelengths with the SKA. For GAIA, tracking N ~ 10^6
quasars over a time of T ~ 1 yr with an angular accuracy of \Delta \theta ~ 10
\mu as would yield a sensitivity level of \Omega_gw ~ (\Delta \theta)^2/(N T^2
H_0^2) ~ 10^-6, which would be comparable with pulsar timing. In this paper we
take a first step toward firming up these estimates by computing in detail the
statistical properties of the angular deflections caused by a stochastic
background. We compute analytically the two point correlation function of the
deflections on the sphere, and the spectrum as a function of frequency and
angular scale. The fluctuations are concentrated at low frequencies (for a
scale invariant stochastic background), and at large angular scales, starting
with the quadrupole. The magnetic-type and electric-type pieces of the
fluctuations have equal amounts of power.Comment: 23 pages, 2 figures, references added and minor text correction
Lagrangian perfect fluids and black hole mechanics
The first law of black hole mechanics (in the form derived by Wald), is
expressed in terms of integrals over surfaces, at the horizon and spatial
infinity, of a stationary, axisymmetric black hole, in a diffeomorphism
invariant Lagrangian theory of gravity. The original statement of the first law
given by Bardeen, Carter and Hawking for an Einstein-perfect fluid system
contained, in addition, volume integrals of the fluid fields, over a spacelike
slice stretching between these two surfaces. When applied to the
Einstein-perfect fluid system, however, Wald's methods yield restricted
results. The reason is that the fluid fields in the Lagrangian of a gravitating
perfect fluid are typically nonstationary. We therefore first derive a first
law-like relation for an arbitrary Lagrangian metric theory of gravity coupled
to arbitrary Lagrangian matter fields, requiring only that the metric field be
stationary. This relation includes a volume integral of matter fields over a
spacelike slice between the black hole horizon and spatial infinity, and
reduces to the first law originally derived by Bardeen, Carter and Hawking when
the theory is general relativity coupled to a perfect fluid. We also consider a
specific Lagrangian formulation for an isentropic perfect fluid given by
Carter, and directly apply Wald's analysis. The resulting first law contains
only surface integrals at the black hole horizon and spatial infinity, but this
relation is much more restrictive in its allowed fluid configurations and
perturbations than that given by Bardeen, Carter and Hawking. In the Appendix,
we use the symplectic structure of the Einstein-perfect fluid system to derive
a conserved current for perturbations of this system: this current reduces to
one derived ab initio for this system by Chandrasekhar and Ferrari.Comment: 26 pages LaTeX-2
Asymptotically (anti) de Sitter Black Holes and Wormholes with a Self Interacting Scalar Field in Four Dimensions
The aim of this paper is to report on the existence of a wide variety of
exact solutions, ranging from black holes to wormholes, when a conformally
coupled scalar field with a self interacting potential containing a linear, a
cubic and a quartic self interaction is taken as a source of the
energy-momentum tensor, in the Einstein theory with a cosmological constant.
Among all the solutions there are two particularly interesting. On the one
hand, the spherically symmetric black holes when the cosmological constant is
positive; they are shown to be everywhere regular, namely there is no
singularity neither inside nor outside the event horizon. On the other hand,
there are spherically symmetric and topological wormholes that connect two
asymptotically (anti) de Sitter regions with a different value for the
cosmological constant. The regular black holes and the wormholes are supported
by everywhere regular scalar field configurations.Comment: Final versio
Constants of Geodesic Motion in Higher-Dimensional Black-Hole Spacetimes
In [arXiv:hep-th/0611083] we announced the complete integrability of geodesic
motion in the general higher-dimensional rotating black-hole spacetimes. In the
present paper we prove all the necessary steps leading to this conclusion. In
particular, we demonstrate the independence of the constants of motion and the
fact that they Poisson commute. The relation to a different set of constants of
motion constructed in [arXiv:hep-th/0612029] is also briefly discussed.Comment: 8 pages, no figure
Bose-Einstein condensate and Spontaneous Breaking of Conformal Symmetry on Killing Horizons
Local scalar QFT (in Weyl algebraic approach) is constructed on degenerate
semi-Riemannian manifolds corresponding to Killing horizons in spacetime.
Covariance properties of the -algebra of observables with respect to the
conformal group PSL(2,\bR) are studied.It is shown that, in addition to the
state studied by Guido, Longo, Roberts and Verch for bifurcated Killing
horizons, which is conformally invariant and KMS at Hawking temperature with
respect to the Killing flow and defines a conformal net of von Neumann
algebras, there is a further wide class of algebraic (coherent) states
representing spontaneous breaking of PSL(2,\bR) symmetry. This class is
labeled by functions in a suitable Hilbert space and their GNS representations
enjoy remarkable properties. The states are non equivalent extremal KMS states
at Hawking temperature with respect to the residual one-parameter subgroup of
PSL(2,\bR) associated with the Killing flow. The KMS property is valid for
the two local sub algebras of observables uniquely determined by covariance and
invariance under the residual symmetry unitarily represented. These algebras
rely on the physical region of the manifold corresponding to a Killing horizon
cleaned up by removing the unphysical points at infinity (necessary to describe
the whole PSL(2,\bR) action).Each of the found states can be interpreted as a
different thermodynamic phase, containing Bose-Einstein condensate,for the
considered quantum field. It is finally suggested that the found states could
describe different black holes.Comment: 36 pages, 1 figure. Formula of condensate energy density modified.
Accepted for pubblication in Journal of Mathematical Physic
FIRE-PROTECTION WITH ALKALI-ACTIVATED CEMENT BINDER
Fire resistance of unprotected steel structures is very low and steel elements must be protected from fire. One possibility is to create a protective layer of a cement-based material. Most types of cement have a low resistance to high temperatures, reducing mechanical properties. In flammability tests, cement activated with alkaline compounds showed better properties compared to conventional types of cement. This paper represents the determination of the properties of two H-Cement mortars with experlite or fireclay sand. Experiments carried out in a small kiln simulating a 1D load showed differences between elements in terms of heat transfer to the tested elements. The calculation model created to predict the course of the experiments has been validated and the unknown properties of the material have been calculated based on the data collected. The samples were tested in a small fire furnace. Finally, the thermal conductivity pattern was determined depending on the temperature
The Power Spectrum in de Sitter Inflation, Revisited
We find that the amplitude of quantum fluctuations of the invariant de Sitter
vacuum coincides exactly with that of the vacuum of a comoving observer for a
massless scalar (inflaton) field. We propose redefining the actual physical
power spectrum as the difference between the amplitudes of the above vacua. An
inertial particle detector continues to observe the Gibbons-Hawking
temperature. However, although the resulting power spectrum is still
scale-free, its amplitude can be drastically reduced since now, instead of the
Hubble's scale at the inflationary period, it is determined by the square of
the mass of the inflaton fluctuation field.Comment: 4 page
Are stealth scalar fields stable?
Non-gravitating (stealth) scalar fields associated with Minkowski space in
scalar-tensor gravity are examined. Analytical solutions for both non-minimally
coupled scalar field theory and for Brans-Dicke gravity are studied and their
stability with respect to tensor perturbations is assessed using a covariant
and gauge-invariant formalism developed for alternative gravity. For
Brans-Dicke solutions, the stability with respect to homogeneous perturbations
is also studied. There are regions of parameter space corresponding to
stability and other regions corresponding to instability.Comment: 10 pages, 1 table, no figures, to appear in Phys. Rev,
Cosmological perturbations from stochastic gravity
In inflationary cosmological models driven by an inflaton field the origin of
the primordial inhomogeneities which are responsible for large scale structure
formation are the quantum fluctuations of the inflaton field. These are usually
computed using the standard theory of cosmological perturbations, where both
the gravitational and the inflaton fields are linearly perturbed and quantized.
The correlation functions for the primordial metric fluctuations and their
power spectrum are then computed. Here we introduce an alternative procedure
for computing the metric correlations based on the Einstein-Langevin equation
which emerges in the framework of stochastic semiclassical gravity. We show
that the correlation functions for the metric perturbations that follow from
the Einstein-Langevin formalism coincide with those obtained with the usual
quantization procedures when the scalar field perturbations are linearized.
This method is explicitly applied to a simple model of chaotic inflation
consisting of a Robertson-Walker background, which undergoes a quasi-de-Sitter
expansion, minimally coupled to a free massive quantum scalar field. The
technique based on the Einstein-Langevin equation can, however, deal naturally
with the perturbations of the scalar field even beyond the linear
approximation, as is actually required in inflationary models which are not
driven by an inflaton field such as Starobinsky's trace-anomaly driven
inflation or when calculating corrections due to non-linear quantum effects in
the usual inflaton driven models.Comment: 29 pages, REVTeX; minor changes, additional appendix with an
alternative proof of the equivalence between stochastic and quantum
correlation functions as well as an exact argument showing that the
correlation function of curvature perturbations remains constant in time for
superhorizon modes, which clarifies a recent claim in arXiv:0710.5342v
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